Cycles Preview Render takes too long
You have 2 different sampling ratios, available in the Properties menu > Render > Sampling > Samples:


the Preview is the ratio used when you work your scene
the Render is the ratio used when you launch a definitive render of your scene (either single image or animation when you click the Render or Animation button on the top of this Render menu).


You can decrease the Preview quality so that it takes less time to render when you work, for example 3 may be enough.

The Render ratio depends on what you want as a definitive result, you can begin with 100

In the next version of Blender (2.8), the Eevee render engine seems much faster than Cycles, it will generate a high quality image in real time, but the result will be slightly different